The invention pertains to the biological treatment of wastewater to remove pollutants. More specifically, the invention provides a wastewater treatment process and apparatus employing attached cultures of microbiological forms contained in the wastewater that can reduce substantially the quantity of both carbonaceous and nitrogeneous pollutants discharged from a treatment plant into natural receiving waters, such as lakes, rivers, and streams.
Prior art wastewater treatment systems are primarily concerned with removing carbonaceous pollutants from the wastewater and secondarily with the removal of other materials such as soluble nitrogeneous compounds, e.g., ammonia and nitrates. Recently, increased attention has been given to the presence of nitrates in receiving waters, particularly in potable water supplies. It has been shown that excessive nitrate concentration in drinking water can adversely effect the physiology of infants by reducing the oxygencarrying capacity of the bloodstream.
Various governmental bodies have responded to the problem of excessive amounts of nitrogenous compounds in natural receiving waters by regulating the permissible concentration of such compounds in wastewater effluents. The State of Maryland has set a limit of 1 mg/liter of nitrogen in the form of ammonia in wastewater discharged into the Patuxent River; the Potomac Enforcement Conference has recently required the removal of 85 percent of the total nitrogen from all wastewater discharged into receiving waters from the Washington Metropolitan Area; and the State of Illinois, has stipulated that the ammonia-nitrogen concentration in wastewater effluents be reduced to not more than 2.5 mg/liter.
The nitrogenous matter in normal wastewater typically comprises ammonia, a small fraction of intermediate amino compounds, and a large fraction in the form of proteins. The proteinaceous matter in wastewater exists mainly in particulate form and is physically removed, primarily by sedimentation.
Processes for removing ammonia-nitrogen from wastewater follow two general lines of approach: (1) the physio-chemical, and (2) the biological. Physiochemical processes generally have the basic disadvantage of being costly, producing undesirable side effects, generating a concentrated brine for disposal, and in some cases, polluting the air with ammonia vapors. The biological processes on the other hand, promote the natural cycling of nitrogen, i.e., oxidation of the ammonia and denitrification of the resulting nitrates to nitrogen gas.
A preferred biological process for treating wastewater to oxidize carbonaceous matter utilizes rotating biological contactors in the form of plurality of closely spaced, partially submerged bodies, to grow fixed biological slimes. The rotating biological contactors can be in the form of thin disks, drums, cylinders, brushes, etc. A preferred contactor has a discontinuous honeycombtype structure and is disclosed in pending U.S. Pat. application Ser. No. 252,038 filed May 10, 1972.
The partially submerged bodies, are forcibly rotated to alternately expose the organisms on the contact surfaces to the atmosphere for the absorption of oxygen and the wastewater for nutrients.
Such rotating biological contactors, arranged to subject the wastewater to sequential treatment by 4 to 6 stages arranged in series requires about one-third less surface area than would a comparable unit having only a single treatment stage when used for the purpose of removing only carbonaceous matter. Experience has shown that about 400,000 sq. ft. of surface divided into 4 to 6 treatment stages is capable of removing 90 percent of the carbonaceous matter, measured in terms of B.O.D..sub.5 (see Table I for definition) from 1 million gallons per day (M.G.D.) of settled domestic wastewater.
Representative wastewater treatment systems utilizing a multistage configuration of partially submerged rotating biological contactors for the sequential treatment of wastewater are shown and described in Torpey U.S. Pat. No. 3,575,849; El Nagger U.S. Pat. No. 3,335,081; Simpson U.S. Pat. No. 3,466,241; and Hartmann et al. U.S. Pat. No. 3,389,798.
When the concentration of carbonaceous matter in wastewater has been reduced by about 90 percent using the above-described multi-stage system employing rotating biological contactors, a separate and additional multi-staged treatment system is typically used to oxidize the ammonia-nitrogen present in the wastewater. Specific nitrifying organisms attach to and develop on the rotating surfaces of the separate and additional system forming slimes which oxidize the ammonia to nitrates. The subsequent treatment system for biological oxidizing ammonia loses much of the contact area for the nitrifying slimes as a result of predator activity and requires about 1 million sq. ft. of surface to oxidize substantially all the ammonia in one M.G.D. of normal domestic wastewater.